Allogeneic hematopoietic cell transplantation (allo-HCT), as a classic approach for cancer immunotherapy, can cure a variety of hematopoietic malignances such as leukemia, termed as the graft-versus-leukemia (GVL) effect. However, graft-versus-host disease (GVHD) remains the primary case of transplant-related morbidity and mortality, thereby limiting the use of allo-HCT. GVHD develops in two forms: acute and chronic with distinct etiology, pathophysiology, and response to therapeutic regimens. aGVHD is primarily induced by T cells commonly characterized by a type I T-cell response; whereas cGVHD is induced by both T and B cells similar in nature to autoimmune disorders. Despite advances in patient care and pharmacologic prophylaxis strategies, the incidence of GVHD particular cGVHD has not been substantially reduced over the years. Because effective treatment options are very limited beyond steroids, it is urgently required to further understand cGVHD pathogenesis and identify novel therapeutic targets for the prevention and treatment of the devastating disease as a major complication of allo-HCT. Sphingolipids are involved in signaling pathways that mediate cell growth, differentiation, cell death and multiple cell functions. Ceramide is considered at the central hub of sphingolipid metabolism and can be synthesized by de novo mechanism that involves 6 key enzyme ceramide synthases (CerSs). The importance of lipid rafts and ceramide platforms in modulating immune responses of T and B cells has been elucidated. Our preliminary data shows that T cells deficient for CerS6 impaired TCR-signaling, activation, and pathogenecity in the induction of aGVHD. We hypothesize that ceramide synthesis mediated by CerS6 is critical for the activation and function of both T and B cells, and thus CerS6 can a valid therapuetic target for the control of cGVHD. To test the hypothesis, we propose to pursue two Specific Aims: 1) Define the role of CerS6 in T- and B-cell responses in the development of cGVHD after allogeneic HCT; 2) Determine the therapeutic effect of CerS6-inhibitor in the control of cGVHD. If Aims are achieved as expected, the project will set a platform for further studying how sphingolipid metabolism affects adaptive immunity and identify novel molecular targets for manipulating immune response.